Exposure to excessive noise over an extended period may result in a permanent loss of hearing. Noise can also be hazardous as well as objectionable to nearby residents of an industrial plant. In the United States, industrial plants are legally responsible for worker safety, including control and abatement of excessive noise. One of the provisions of the Occupational Safety and Health Act (OSHA) of 1970 relates specifically to the occupational exposure of workers to noise. Under the Noise Control Act of 1972, the Environmental Protection Agency (EPA) was mandated by Congress to establish noise limits protective of public health and welfare with an adequate margin of safety. In addition to federal regulatory efforts, states and cities have set maximum noise levels acceptable at plant property lines. These levels typically vary according to area zoning--heavy industrial, commercial, residential and hospital. With tighter controls now in effect, every effort should be made to eliminate or measurably reduce noise at its source in a logical and systematic manner.
Ideally, the optimum approach to noise control is the prevention (or reduction) of noise at its source before it becomes a problem. However, equipment re-design and/or operational changes necessary to accomplish this are seldom possible.
Effective noise control is normally achieved by means of: (a) isolation, (b) dissipation or (c) a combination of the two. This involves the use and application of sound absorption materials, acoustic shields and barriers (fixed and movable), acoustic enclosures and/or silencers.
Typically, silencers are divided into three distinct categories: (a) the reactive "reflective type, (b) the dissipative "absorptive" type and (c) a combination of these two basic types. The present invention relates to dissipative and combination reflective/dissipative silencers.
The reactive-type silencer is generally restricted to relatively low-frequency applications, such as the intake and exhaust of engines, blowers and compressors. It is largely dependent upon an area discontinuity to reflect sound energy back to the source, and upon the dissipative effect of perforated, ported or slotted tubes for effective broad-band, low-frequency performance. Multi-chamber reactive silencers are available in: (a) straight-through tube arrangements for low-loss, pressure-drop applications and (b) in labyrinth-like, volume-tube configurations where pressure drop is not critical. Acoustic performance is a function of silencer diameter, overall volume and internal design. Absorptive material is not used in a pure reactive-type silencer.
The dissipative-type silencer is essentially a high-frequency, low-pressure drop attenuator. It depends on sound absorbing material to dissipate the sound energy and is usually applied on the intake and exhaust of centrifugal compressors, forced draft fans, gas turbines, steam or process vents and similar equipment. Dissipative silencers are usually straight runs of acoustically lined piping or parallel baffles. Performance depends upon the internal design and the type of absorptive material used. The open flow area ranges from 25 to 75 percent, depending upon the required attenuation and allowable pressure drop.
The acoustic fill in dissipative-type silencers is usually mineral wool, polyester, fiberglass or another durable, inert, vermin-proof, moisture-resistant material. For intake applications, the density of the acoustic fill is typically 21/2 to 4 pounds per cubic foot packed under 10 percent compression to prevent voids. In exhaust service, the density is increased to a minimum of 6 pounds per cubic foot and, depending upon the temperature and unit velocity, the fill is normally protected with one or two wraps of fiberglass cloth or with one wrap of cloth and a stainless steel mesh screen, plus a perforated face sheet.
The combination reactive/dissipative silencer is functionally a reactive silencer with sound absorptive material to provide added high-frequency noise reduction. The perforated, slotted or ported tube applied to the reactive silencer acts as a dissipative element, reducing or eliminating troublesome pass-bands inherent to the basic reactive design. The performance of an effective reactive silencer is a result of both the dissipation and reflection of noise energy.
Silencer design is influenced to a large extent by intended use and application. The focus of this patent is an industrial silencing application where the velocity of the gas stream through the silencer is typically greater than 10,000 feet per minute and the temperature of the gas is usually greater than about 700.degree. F. These operating conditions are typical when designing for high-pressure steam vent safety relief valve silencers and gas turbine engine silencers.
The industry standard for these "high-velocity/high-temperature" applications has been to use an acoustic pack material such as mineral or fiberglass wool that consists of a collection of small individual fibers wrapped in a protective fiberglass cloth or mesh wire. Typically, industrial silencers built for this type of high temperature--high flow application use a combination of an absorptive and reactive design to achieve the necessary noise reduction. Absorptive material is packed between the outer shell of the silencer and a perforated cylinder through which the gas flows. A barrier is then inserted into the flow path in order to redirect the gas flow to the perforated wall while the barrier dissipates the lower frequency sound waves. When the gas flow is redirected into the perforated cylinder, the higher frequency sound waves are absorbed by the acoustic material packed between the perforated cylinder and the outer shell of the unit.
While the above-described design has proven effective for noise reduction, a problem often occurs due to the high velocity and temperatures specific to these silencing applications. The typical absorptive material used for these applications often fragments due to the extreme gas velocity and temperature. There is thus a risk that the fragmented material will be expelled through the perforated holes into the gas stream. This fragmentation also results in a pack density much higher than intended, which can reduce the absorptive quality of the pack material. Finally, the fragmentation leads to a collapse of the material which leaves voids in the cavity that allow sound waves to pass through freely.
Applications involving high velocity--high temperature gas flows such as gas turbine engine silencers have always been vulnerable to pack migration as well. The common solution has been to modify the internal design of the unit and insert physical barriers to prevent the pack material from migrating or being expelled from the cavity. While this method has been successful in preventing the migration of the pack, the pack material is still vulnerable to fragmentation. The resulting "voids" in the cavity along with other changes in the internal design to reduce migration can affect the performance of the unit, which is critical when silencing a gas turbine engine.
What is needed, then, is a new silencer design for high velocity--high temperature gas flow applications that achieves suitable noise reduction and also prevents pack fragmentation and migration.